fix(sessions): Surface failed agent prompt sends to the user

apps/code/src/renderer/desktop-services.ts

@@ -152,8 +152,8 @@ container
   .toConstantValue(new RendererHedgehogModeHost());
 container
   .bind<AgentPromptSender>(AGENT_PROMPT_SENDER)
-  .toConstantValue((taskId, prompt) => {
-    void resolveService<SessionService>(SESSION_SERVICE).sendPrompt(
+  .toConstantValue(async (taskId, prompt) => {
+    await resolveService<SessionService>(SESSION_SERVICE).sendPrompt(
       taskId,
       prompt,
     );

packages/ui/src/features/sessions/agentPromptSender.ts

@@ -3,6 +3,6 @@ import type { ContentBlock } from "@agentclientprotocol/sdk";
 export type AgentPromptSender = (
   taskId: string,
   prompt: string | ContentBlock[],
-) => void;
+) => Promise<void>;
 
 export const AGENT_PROMPT_SENDER = Symbol.for("posthog.ui.AgentPromptSender");

packages/ui/src/features/sessions/sendPromptToAgent.test.ts

@@ -0,0 +1,69 @@
+import { toast } from "@posthog/ui/primitives/toast";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { AGENT_PROMPT_SENDER } from "./agentPromptSender";
+import { sendPromptToAgent } from "./sendPromptToAgent";
+
+const { mockSender } = vi.hoisted(() => ({ mockSender: vi.fn() }));
+
+vi.mock("@posthog/ui/primitives/toast", () => ({
+  toast: { error: vi.fn(), info: vi.fn(), success: vi.fn() },
+}));
+
+vi.mock("@posthog/di/container", () => ({
+  resolveService: (token: unknown) => {
+    if (token === AGENT_PROMPT_SENDER) return mockSender;
+    throw new Error(`resolveService: unmocked token ${String(token)}`);
+  },
+}));
+
+vi.mock("../code-review/reviewNavigationStore", () => ({
+  useReviewNavigationStore: {
+    getState: () => ({ getReviewMode: () => "split", setReviewMode: vi.fn() }),
+  },
+}));
+
+vi.mock("../panels/panelLayoutStore", () => ({
+  usePanelLayoutStore: {
+    getState: () => ({ taskLayouts: {}, setActiveTab: vi.fn() }),
+  },
+}));
+
+describe("sendPromptToAgent", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("surfaces a rejected send as an error toast", async () => {
+    mockSender.mockRejectedValueOnce(
+      new Error("Agent server is not reachable"),
+    );
+
+    sendPromptToAgent("task-1", "hello");
+
+    await vi.waitFor(() =>
+      expect(toast.error).toHaveBeenCalledWith("Agent server is not reachable"),
+    );
+  });
+
+  it("falls back to a generic message when the rejection is not an Error", async () => {
+    mockSender.mockRejectedValueOnce("boom");
+
+    sendPromptToAgent("task-1", "hello");
+
+    await vi.waitFor(() =>
+      expect(toast.error).toHaveBeenCalledWith(
+        "Failed to send your message to the agent. Please try again.",
+      ),
+    );
+  });
+
+  it("does not toast when the send resolves", async () => {
+    mockSender.mockResolvedValueOnce(undefined);
+
+    sendPromptToAgent("task-1", "hello");
+
+    await Promise.resolve();
+    await Promise.resolve();
+    expect(toast.error).not.toHaveBeenCalled();
+  });
+});

packages/ui/src/features/sessions/sendPromptToAgent.ts

@@ -1,5 +1,6 @@
 import type { ContentBlock } from "@agentclientprotocol/sdk";
 import { resolveService } from "@posthog/di/container";
+import { toast } from "@posthog/ui/primitives/toast";
 import { useReviewNavigationStore } from "../code-review/reviewNavigationStore";
 import { DEFAULT_TAB_IDS } from "../panels/panelConstants";
 import { usePanelLayoutStore } from "../panels/panelLayoutStore";
@@ -17,7 +18,19 @@ export function sendPromptToAgent(
   taskId: string,
   prompt: string | ContentBlock[],
 ): void {
-  resolveService<AgentPromptSender>(AGENT_PROMPT_SENDER)(taskId, prompt);
+  // Button/review/skill-initiated prompts are fire-and-forget, but a rejected
+  // send (auth failure, sandbox unreachable, agent process died) must still be
+  // surfaced — otherwise the turn just shows "Generated in Xs" with no reply.
+  void resolveService<AgentPromptSender>(AGENT_PROMPT_SENDER)(
+    taskId,
+    prompt,
+  ).catch((error: unknown) => {
+    toast.error(
+      error instanceof Error
+        ? error.message
+        : "Failed to send your message to the agent. Please try again.",
+    );
+  });
 
   const { getReviewMode, setReviewMode } = useReviewNavigationStore.getState();
   if (getReviewMode(taskId) === "expanded") {